This tiny Supernova made by Jack Krijnen is the smallest 18-piece burr I've seen : only 3.6 cm !

INTRO

18-piece burrs are a bit mysterious. The Van de Poel, a design from 1953 nicknamed "The Granfather of 6x6x6" by Rob Stegman, was described as the most difficult of all construction puzzles by Van Delft and Botermans in their book "Creative Puzzles of the World".
It was necessary to perform 3 moves to get the first piece out !
Now, the record, held by Supernova, is 166 moves.

Because of this, the real difficulty level of this kind of puzzles is mostly unknown. It is admitted that the goal is to start from the assembled puzzle, and to disassemble it, but even that is subject to debate. Some puzzlist carefully write the position and order of insertion of each piece while they disassemble the puzzle in order to be able to rebuild it. Other rely on the Burrtools software to give them the right sequence.
Two 18-piece designs were actually arranged as puzzles to disassemble and/or reassemble : Lange Wapper 14 v2 and Tiros v2. And only the later was commercialized.

Muff and Supernova are two of Alfons Eyckmans' creations, and are among the four 18-piece designs needing the highest number of moves to get one piece out of the puzzle : 93 and 166 respectively.
Supernova was designed in collaboration with Jack Krijnen, who also designed the Century (100 moves). Alfons has just created a fourth one featuring 130 moves.

There are so many designs published (Burrly Sane, Barones, Excelsior, Tiros etc), and so many more that are unpublished, that it is difficult to sort out which ones are minor variations and which ones are really different. Here is a small "phylogenetic" tree showing which designs are close to each other and which ones are different, among the few ones that are relevant for this topic.

BSWW = Burrly Sane for Wood Workers
BSP = Burrly Sane for Professionals
BSEP = Burrly Sane for Extreme Puzzlers
The numbers are the moves needed to get the fist piece out of the puzzle.

Condor's Peeper, Muff and Supernova have a common origin, but stand on different branches of the tree. When you have solved one, it doesn't help you to solve another. It may even lure you into false tracks, because some things look similar, while they are not. It is only when we get to know them quite well that we realize that there are strong similarities in their structure.
Puzzles standing on the same branch of the tree share a common part in their solution. Once you know one of them well enough, all you have to do to solve another is to find the differences.
Century stands completely apart. It has nothing in common with the others. I don't know anything yet about Alfons' last creation, save that it features 130 moves and is not on the same branch as Supernova.

I put Supernova on top of the tree because it has the highest number of moves, and is indeed the one with the most complexe disassembling sequence. However, the record for the highest total number of moves is still held by Burrly Sane for Extreme Puzzlers, with 209 moves to completely disassemble the puzzle, while 201 are enough to completely disassemble Supernova.

Why didn't I include Century in this review ? First because I don't know it well enough, and second, because I think that only Muff may pretend to be as difficult as Supernova. We will see why below.

MAKING OF

We made the Muff and the Supernova the same day with Maurice Vigouroux. We used larger sticks than usual, with a cross-section of 25x25 mm instead of 20x20. Me because I prefer handling large puzzles, and the mechnical accuracy is better, Maurice because he wanted to have outstanding objects in his showroom. Maurice chose the most beautiful woods he had. Here is the result :

His Supernova is in tulipwood, kingwood and ziricote, and his Muff in ziricote, morado (Santos rosewood) and padauk. To his surprise, when he asked me to choose the woods I would like, I said maple, cherry and walnut !
Actually, as Supernova was expected to be the best 18-piece burr in my collection, and I planned to play with it a lot, I wanted it to have a perfect mechanical behaviour, and I know from experience that high level burrs behave better with soft european woods. Hard exotic woods are more difficult to work with because they are unforgiving with small errors. When a piece is 0.1 mm too short, with hard woods, it feels hard to move. With softer woods, it still slides properly.
For the Muff, I replaced the walnut with coconut (red palm) because I wanted the two puzzles to have a different look, and especially because I had to use this small beautiful bit of coconut that I had brought from Spain, where I got it from my cosin, who brought it from Thailand.

The result met our expectations. Maurice have two Rolls-Royce, and I have two formula one

I assembled the first puzzles made following the solution given by Burrtools. I did it several times, in fact, because there were three of them, and sometimes a given move was a bit too tight, and I had to identify the faulty pieces, disassemble everything, have the pieces adjusted by Maurice, and reassemble everything...

Therefore, back home, sitting quietly with the puzzles in my hand ready to be solved, I already had a general idea about their respective solutions.

SUPERNOVA

It is difficult for me to judge the difficulty level of Supernova, because I already knew by heart the solution of Tiros when I disassembled it. I was still a great experience because I had to re-use all that I knew while facing new challenges standing in my way. Aaron Davila, who also solved the two puzzles describes it as a sort of "familiar maze with extra twists and turns, a delightful break from the monotony of the Tiros solution (and other Tiros family members). It keeps my attention and is entertaining. The "brick wall" at the end is quite complicated and harder than Tiros.".

The "brickwall" that he refers about is a stage when, after having progressed in the disassembly using clear sequences of moves, one gets lost in a configuration whith much freedom, but no clue about what could be useful to try in order to go further.

MUFF

Muff features a frightening number of moves in the complete disassembling sequence : 93+18+18+7+4+10+6+10+6... with a total of 182. Tiros, in comparison, has a total of 189, which is only 7 more.
Usually, if 10 moves are to be found for the first piece, it is not difficult. For the second piece, it is more complicated. For pieces after the third one, it may become a real nightmare, because the puzzle have more and more freedom. Finding the only possible sequence of moves in a fixed structure is easy. Finding the only righ sequence among hundred of possible ones is another story !

Solving Supernova, Aaron and I both started from the assembled state because we feared that starting from scratch with the disassembled pieces to put together would have been too difficult.

While Supernova has 6 possible assemblies and only one that is possible to put together, Muff has one possible assembly.
I thus started with my Muff disassembled and looked for the position that each piece should have in the final shape. That's something I already did with Tiros and Lange Wapper 14. For the first one, I managed it starting from the 6 central pieces, then finding the position of the 12 outer ones. This method didn't work with Lange Wapper 14, and I had to look at the most massive pieces in order to find a starting point.

But with Muff, none of these methods worked. Many many positions seem possible and I didn't find any clue that would help in finding the right one. I eventually gave up, reassembled the puzzle following the solution in order to try to disassemble it by myself. In order not to have too many clues from the solution, I took care, each time I had to look at the solution, to do it in an impractical orientation, hoping that, once I have the puzzle in a good orientation, I couldn't remember what I did.

That worked well. After three hours trying to disassemble Muff by myself, I couldn't get anywhere. Since I was making absolutely no progress, I forfeited and had a look at the solution to find what I was missing. The right move didn't look especially well hidden. It's just that it looked like a completely unuseful confuguration among many others.
Then I went on by myself... only to fail again ! I had a second look at the solution to to find another sequence. Then at last I could finish the disassembling.

Though it was a very hard challenge, I was not so excited about it. I found nothing special in the solution. No "aha" moment. When I failed to disassemble Burrly Sane for Woodworkers, and looked at the solution, it was illuminating. I said to myself : nooooooooo ! It could move!!! It could move and I didn't even push it !!!
But discovering what I missed in the Muff didn't produce the same effect. I saw that the right sequence was to move all the pieces that I had already tried, but in a different order, and for an obscure purpose. I told myself : So it was that ? Yep, it allows that piece to pass over that one without hitting this one. Ok, and now what ?

Alfons had warned me that, in his opinion, Muff was a very hard puzzle. But unlike what I feared, it's not the moves after the first piece that were difficult. It is the first 93-moves sequence !

OTHER OPINIONS

I shared my thoughts with Aaron, who managed to disassemble Muff without any help. Comparing Muff with Tiros, he finds that Tiros has more dead ends, and that the "brickwall" near the end of the disassembling sequence makes it very difficult. He finds Muff to "flow in a more linear fashion, this is because it seems to have less movement more pieces are locked up". He also finds that the brickwall is lacking, making the removal of the first piece easier.
What he mostly likes in Muff is that it doesn't behave like we expect it to. He describes it as a "wolf in sheeps clothing". It looks and behave like Tiros, but isn't.
Overall he finds it as difficult as Tiros.

I was a bit surprised, because my opinion was the exact opposite : I found Tiros quite linear, and Muff full of dead ends !
We compared our points of view. Aaron views these kind of puzzles from a strategic point of view. "I gauge the puzzle on how much studying and foresight is required to understand the mechanics of the puzzle. I also look as to how many "dead end" pathways to nowhere there are." he says.

My own way of seeing high level burrs include a different thing : I like when the solution can be divided into smaller sequences. In Tiros, for example, the sequence that consists in moves 2, 3 and 4 in the disassembling sequence appears 15 times in the solution. And several sequences, that often use the 2-3-4 sequence themselves, eventually move only one piece :
Moves 1 to 13 : one piece is moved.
Moves 14 to 40 : another piece has moved.
And then you play moves 13 to 1 in the reverse order, which gives :
Moves 1 to 53 : one piece has moved !

This makes the puzzle closer to both "sequencial discovery" and "twisty" puzzles categories : after trying some moves at random, we can discover a complete sequence that allows to move one piece without changing anything else (like in twisty puzzles). Then, later, we can use this sequence again as a tool in order to move that piece another time (like in a sequencial discovery puzzle).
That's my preferred part in high level burrs, because it gives a sense of progression, that we are not turning into circles, or just walking blindly in a maze until mere chance allows us to find the exit.

THE CHALLENGE

Since we were disagreeing about which puzzle had the more dead ends, I proposed a little challenge to Aaron : I bet that I could find a longer dead end in Muff that he would ever find in Tiros.
We defined a dead end as a configuration of the puzzle that does not belong to the shortest path from the assembled state to the disassembled state. The lenght of such a dead end is the shortest path from this configuration to any configuration that is part of the solution.

Aaron proposed to improve the challenge and count how many dead ends there are, ignoring the single false moves and short dead ends. That would help making a kind "map of difficulty" for each puzzle. I accepted, but warned him that it may take a lot of time before I can master Muff well enough.

I began 3 months later. I brought Muff with me during a trip in España... and failed to disassemble it again ! Fortunately it didn't prevent me to start tracking some dead ends. In order to do so, I made a table where each column is a puzzle piece that is involved in the main sequence. A configuration is noted in a row, with numbers that represents the offset of each piece relative to its normal position.

Back to home, I checked the solution again, and completed my list. 3 more monthes later, I announced to Aaron that I had found 14 dead ends around Muff's 93 moves sequence. He replied that he found 7 in Tiros. One of them was an alternative path from a configuration to another that was 32 moves long. However, it was never more than 7 moves away from the main path, while I found three positions for Muff that were 13, 16, and 16 moves away from the main path respectively. I also found a configuration in Tiros that is 14 moves away from any configuration used in the solution.

The funny thing is that this configuration is reached starting from the puzzle completely assembled, and moving away from the solution, which allows to put the puzzle in a configuration that needs at least 14 + 150 = 164 moves to get one piece out.
With Supernova, the same dead end is 16 moves long, which makes a 182-moves object. For Muff, the first dead end is also in the same case and turns it into a 106 moves puzzle.

According to these results, I think that Muff and Supernova might be the two 18-piece burrs that are the most difficult to disassemble so far.

Pio2001 wrote:The Van de Poel, a design from 1953 nicknamed "The Granfather of 6x6x6" by Rob Stegman, was described as the most difficult of all construction puzzles by Van Delft and Botermans in their book "Creative Puzzles of the World".
It was necessary to perform 3 moves to get the first piece out !

The Van de Poel puzzle has 2 solutions for 30 assemblies. I don't own this puzzle, so I don't know if there is a possibility to find these solutions without a time-consuming enumeration of all possible configurations (enumeration that may be inhuman with 18 pieces times 20 assemblies).

It can thus indeed be extremely difficult as a construction puzzle. Maybe more than Supernova or Muff, considered here as disentanglement puzzles